Electrochemical cells, e.g., fuel cells, are typically formed with an ionic membrane with electrocatalysts disposed on the membrane surfaces to catalyze the appropriate oxidation and reduction reactions occurring on the surfaces. Conductive gas permeable surfaces, typically carbon cloth, contact the membrane surfaces to establish an external current flow. Anode and cathode plates then contact the conductive surfaces to provide external electrical connections. The anode and cathode plates also provide for the flow of fuel and oxidizing gases, respectively, over the membrane surfaces. The individual electrochemical cells are typically connected in series to provide cell stacks that obtain useful power output.
In a cell stack, the anode plate of one cell is also the cathode plate of an adjacent cell, i.e., a bipolar plate. Bipolar plates are also structural components of a cell stack since the cells are typically subject to compression forces that maintain the entire assembly internally sealed and with good electrical contact along the series of cells. In addition, bipolar plates separate the fuel and oxidizing reaction gases on opposite sides of the plate. Such bipolar plates are often formed of electrically conductive coated solid metals, carbon, or graphite/graphite composites that must be machined to provide channels for the required flow fields on both sides of a plate and provide a minimum thickness for structural support. These plates are expensive to produce since the channels must be machined and are heavy components of the resulting cell stack.
Although corrosion is an important concern, metal is of interest for bipolar plates because of its potential for low-cost manufacture. One approach to protecting metal hardware from corrosion is to use coatings, such as noble metals, or electrically conductive passivation layers, such as titanium nitride or silicon carbide. Another approach is to use uncoated metal alloys that allow a certain amount of fuel cell operation before the cell performance is unduly affected. For example, we have shown that a fuel cell can be operated using untreated 316 stainless steel (316 SS) for up to 2,000 hours before the performance degrades (U. S. Pat. No. 5,798,187 issued Aug. 25, 1998).
The primary mechanism for the degradation is the uptake of metal ions by the polymer electrolyte membrane, which affects the ionic conductivity of the electrolyte. In addition, the metal ions that make their way to the catalyst layer may possibly alter the activity of the catalyst. A number of other "stainless steel" alloys are more noble than the 316 SS, yet still relatively low-cost, and thus promise to extend the useful lifetime. However, 316 SS has been used despite the durability limitations because of its ready availability in numerous forms and foil thicknesses.
Beyond standard machining, which is obviously too expensive for mass production, flow-field channels can be formed into metal plates by a number of potentially lower cost methods, such as chemical etching, pressure jet machining, etc. While these processes can provide a lower cost, a fair amount of potentially expensive material is wasted and the resulting plates are likely to be heavy (unless, of course, aluminum or titanium are used).
Preferably, a relatively thin metal foil could be stamped or otherwise formed into a thin convoluted structure that defines the flow-field channels on either side. The simplest manifestation would be a corrugated plate. For example, U.S. Pat. No. 4,755,272, issued Jul. 5, 1988, teaches a corrugated, electrically conductive sheet for use as a bipolar plate, where one set of channels defined by the corrugations is adjacent the anode and the other set of channels is adjacent the cathode.
Some difficulties arise, however, with the corrugated configuration. It is awkward to manifold the anode and cathode reactant flows to the opposing sides of the corrugated sheet. More importantly, there are practical limitations to this approach with polymer electrolyte fuel cells. Compared to other types of fuel cells and industrial polymer electrolyte reactors, the performance of the polymer electrolyte fuel cell is substantially enhanced with the provision of high clamping pressures.
In conventional flow-field designs, the membrane/electrode assembly (MEA) must span the flow channels. The clamping force on the MEA becomes more uniform and effective as the channels become narrower. However, there are practical limitations due to excessive pressure drops, ease of plate fabrication, ease of water removal, etc. As a result, most conventional flow-field designs use channels that are on the order of only 1 mm in width and depth. One consideration on the depth of the channel is the thickness of the bipolar plate that is required to accommodate the channels, because the channel depths determine the minimum thickness of the plate (minimal plate thicknesses are desirable to maximize stack power densities).
These fine structures pose a challenge for conventional metal forming processes such as stamping or hydroforming. Foils that are thin enough to be readily formed into the requisite intricate structures can not easily provide unsupported structures that can withstand the high clamping pressures desirable for polymer electrolyte fuel cells without deforming or collapsing.
It is thus an objective of this invention to provide a bipolar plate with the structural integrity to withstand high clamping pressure with the use of metal foils that are thin enough to be readily stamped, pleated, or otherwise formed.
This is accomplished in this invention with the use of a structural element, preferably of a plastic material, that supports the metal foils and also defines the sealing and manifolding areas. The result is also a lightweight, thin and durable bipolar plate.
Both the cost and the weight of the unit cells are important for a number of fuel cell stack applications such as transportation or portable power. Consequently, the present invention is directed to a lightweight, inexpensive bipolar plate.
While different in materials and configuration from the present invention, a plastic/metal combination bipolar plate is also taught by U.S. Pat. No. 5,798,188, issued Aug. 25, 1998. Here, the bipolar plates are formed of molded polymer projections that are mounted on a flat conductive plate to form oxidant and fuel flow channels on both sides of the plate without any need to machine the channels. The polymer projections and exposed areas of the conductive plate are coated with a conductive material to form the anode and cathode electrodes.
Various objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.